Gas discharge lamps are commonly known, so an elaborate discussion of the design of a gas discharge lamp is not needed here. Suffice it to say that a gas discharge lamp comprises two electrodes located in a closed vessel filled with an ionizable gas or vapor. The vessel is typically quartz or a ceramic, specifically polycrystalline alumina (PCA). The electrodes are arranged at a certain distance from each other, and during operation an electric arc is maintained between those electrodes.
A gas discharge lamp can be powered by an electronic driver. Electronic drivers are commonly known, so an elaborate discussion of the design of electronic drivers is not needed here. In a typical design, the driver produces a commutating current that is applied to the lamp, resulting in a lamp voltage to develop over the lamp.
Drivers are typically designed to make the lamp current follow a setpoint curve, which in the simplest embodiment involves constant current magnitude; however, depending on for instance lamp type and lamp age, the driver may employ corrective current measures. Further, although gas discharge lamps have long lifetimes in the order of 10.000 hours, the lifetime of a gas discharge lamp is finite. At the end of its life, a gas discharge lamp may show undesirable properties, the most dramatic one being non-passive failure. Thus, it is desirable for the lamp driver to be able to determine lamp type and/or lamp condition, and for instance to be able to switch off a lamp if it turns out that the lamp is approaching the end of its lifetime (EOL). Further, it would be desirable to be able to predict the remaining lifetime.
International patent application WO 2005/074010 discloses a method for investigating a condition of a high-pressure gas discharge lamp. The lamp is operated with a steady-state low-frequency square-wave current signal, the frequency being 90 Hz in the disclosure, the lamp being a 100 W white HPS lamp. A short current pulse is superimposed on the steady-state current signal, the current pulse having a duration of 1.4 ms. In response to this current pulse, the lamp voltage shows a characteristic step (positive or negative) followed by a characteristic decay (negative or positive, respectively) to a substantially constant level. The characteristic decay has a characteristic decay time which can be determined, and which is described as generally varying in the range between about 1 μs and about 1.5 ms. The document describes that a faulty lamp condition, such as a too high color temperature or a too low color temperature, is correlated to the duration of the decay time, such that it is possible to determine the characteristic decay time in order to find whether the lamp property concerned, i.e. the color temperature in this example, is within or outside specification. Then, having found that a lamp property of a specific lamp is outside the operative range, it is possible to take precautionary measurements by switching off the lamp, or it is possible to change the operative conditions (the document discloses the use of additional current components) in order to change the specific lamp property concerned.
In any case, the document discloses that the lamp voltage response to a current step contains at least one parameter (i.e. decay time) that is indicative of a lamp condition or lamp property, which parameter can be measured, compared to a reference value, and corrective or protective measurements can be taken on the basis of the outcome of such comparison.